Method of making an easy opening container wall

ABSTRACT

This invention relates to easy-opening cans of the type commonly filled with beverages and is directed to a number of problems that relate to the fact that the tops of beverage-filled cans are inevitably of outwardly bowed or domed configuration.

United States Patent H 13,ss3,34s

Inventor Omar I... Brown Dayton, Ohio Appl. No. 736,870

Filed Mar. 29, 1968 Patented June 8, 1971 Assignee Erma! C. Frau Dayton,Ohio Division of Ser. No. 565,538, July 15, 1966, Pat. No. 3,428,210.

METHOD OF MAKING AN EASY OPENING CONTAINER WALL 8 Claims, 23 DrawingFigs.

U.S. Cl 113/121, 113/15, 29/522 Int. Cl B2ld 51/00 [50] Field of Search220/54; 113/121, 121 A, 116 FF, 116 CC, 1200; 29/509,

[56] References Cited UNITED STATES PATENTS 3,291,336 12/1966 Fraze220/54 3,251,503 5/1966 Bozek..... 220/54 3,404,800 10/1968 Arfert.....220/54 3,254,790 6/1966 Dunn 220/54 3,366,086 l/1968 Fraze 113/121Primary ExaminerRichard J. Herbst Assistant ExaminerMichael J. KeenanAttorney-Smyth, Roston & Pavitt ABSTRACT: This invention relates toeasy-opening cans of the type commonly filled with beverages and isdirected to a number of problems that relate to the fact that the topsof beverage-filled cans are inevitably of outwardly bowed or domedconfiguration.

PATENTEUJUN slsm 3583,3 18

SHLET 1 BF 6 IA/VINTOA- I 0014'- A. drawn JITOR/VEFI PATENTED Jun 8l97l35 3134 sum 5 OF 6 17% 2 fieep Jrare A K X METHOD OF MAKING AN EASYOPENING CONTAINER WALL This application is a division of Ser. No.565,538, now US. Pat. No. 3,428,2lfiled July 15, 1966.

Some of the problems with which the invention deals stem from the factthat no part of the top of a filled can should protrude beyond the rimof the can to interfere with stacking and handling of filled cans. lnother words, all of the structure of the top end wall of a filledeasy-opening can must be confined to the shallow head space that isdefined by the cylindrical rim flange of the can top.

lfthe top of a filled can bows outward excessively, the depth of the rimflange must be increased accordingly with consequent increase in costper can in a situation where an exceedingly small fraction of a cent percan looms large in a mass production total. If the top of a filled canbows outward moderately but a tab for severing a tear strip of the cantop protrudes excessively or a hollow rivet for anchoring the tab to thetear strip protrudes excessively, again a costly increase in the headspace becomes mandatory. lf it is possible to reduce the head space of afilled can, the cost of the can may be correspondingly reduced withoutreducing the capacity ofthe can or, instead, the capacity of the can maybe increased without increasing the cost of the can.

Any measures that are taken to reduce the outward bowing of a filledeasy-opening can must take into consideration the metal-spreading effectof certain fabrication steps. Notably, the forming of a hollow rivet inthe tear strip has a spreading effect on the sheet metal which is quitepronounced and especially so if a highly desirable method of forming therivet is employed which involves substantial radial extrusion of themetal in an annular zone around the hollow rivet. The scoring of themetal to form the tear strip also has a spreading effect because thepenetration of the sheet metal by the scoring tool displaces the metalin opposite directions.

The spreading of the sheet metal by these two fabrication steps, ineffect, increases the area of the can top and the excess metal causesthe can top to buckle or warp in an irregular manner. Such a can top isbistable in that the buckling of the sheet metal out of the plane of thecan top is predominately in one direction and may be reversed indirection with a snap action.

The cans are filled with the liquid with the cans in upsidedownposition, the can top being lowermost, and the last step is theassembling of the can bottom to the cylindrical body. During the fillingoperation the buckle of the can top which is lowermost may suddenlyreverse or flip with a snap action that causes some of the liquid tospill from the can with consequent reduction of the can content. Suchtroublesome cans are commonly termed flippers.

Later when substantial fluid pressure develops in the filled can, thepressure causes the can top to bow outward by taking up the slack thatis available in the buckle of the sheet metal. Thus the extent to whichthe top of the filled can is bowed outward by internal pressure dependsupon the extent to which the prior steps of fabricating the can top havemade available excess metal to permit the outwardly bowed configuration.

The present invention reduces the outward bowing of a can top byreducing the extent to which the fabrication steps make excess metalavailable for the bowing. The invention also, in effect, reinforces thecan top to resist the bulging of the sheet metal by the fluid pressure.In addition, the invention substan tially eliminates buckling or warp ofthe can top and thus eliminates flippers."

With reference to reducing the available excess metal that is created ina can top by a fabrication step, it is old in the art to offset thesheet metal of a can top in various ways to take up metal after therivet forming and scoring operations are completed and thus contract tosome degree the buckled sheet metal. Both inwardly protruding ribs andoutwardly protruding ribs have been formed in can tops for this purposeof reducing the buckle. The present invention, however, is based on thediscovery that superior end results may be obtained by offsetting thesheet metal simultaneously with any fabrication step that tends tospread the sheet metal. lt has been found that once a buckled conditionis created in a can top, offsetting the sheet metal to form ribs or thelike has limited corrective effect on the buckle, but on the other hand,offsetting the sheet metal concurrently with such a fabrication step ishighly effective to counteract buckling ofthe metal.

One reason for it being difficult to reduce a previously created buckledstate by offsetting the sheet metal is that the buckled configurationper se creates resistance to remedial shifting of the sheet metal.Apparently another reason is that the stressing of the metal by thefabrication steps stiffens the sheet metal by workhardening and byorientation of the grain of the metal. The most important difference,however, between remedial action to correct a buckling that is alreadyformed and preventative action concurrent with a metalspreadingfabrication step is that, in the first instance, the sheet metal is in astatic state when the counteracting step is taken and, in the secondinstance, the sheet metal is in a favorable dynamic state.

Where an operation on the sheet metal of the can top pushes the metal inopposite directions from a processing zone, the invention takesadvantage of this fact by simultaneously pulling the metal in the sameopposite directions in the two corresponding offset zones. Thus twoforces instead of one force act on a particle of metal simultaneously inthe same direction, one force tending to buckle the sheet metal and theother concurrent force negating the tendency to buckle.

ln the fabrication of a prevailing type of easy opening cans, arivet-forming operation is carried out first, and subsequently the cantop is scored to form the tear strip. A feature of the invention is themanner in which the successive buckling tendencies of these twooperations are met by successive simultaneous countermeasures.

In one practice of the invention offset ribs are formed in the sheetmetal simultaneously with the rivetforming operation to counteract theconcurrent buckling tendency and subsequently the same ribs are enlargedconcurrently with the scoring operation to again counteract a bucklingtendency. In a second practice of the invention, rib-forming dies actdirectly on the sheet metal to form ribs during the rivet-formingoperation,just as in the first practice ofthe invention. During thesubsequent scoring operation, however, the dies merely clear thepreviously formed ribs but the sheet metal is free to slide underguidance away from the scoring zone towards the ribs to enlarge theribs.

The invention also teaches that advantage may be taken of the fact thatif two laterally spaced ribs are formed simultaneously by offsetting thesheet metal, the sheet metal between the two ribs is placed undertension. in both practices of the invention the two ribs that are offsetin the metal during the rivet-forming operation are orientated tostraddle the intended location of the tear strip so that the area of theintended tear strip is placed under transverse tension and a part of thedisplacement of sheet metal by the subsequent scoring step serves merelyto relieve this tension instead of causing the sheet metal to buckle.

The ribs that are formed in the can top to forestall buckling areadvantageous in that they stiffen the sheet metal to resist subsequentoutward bowing of the can top by fluid pressure. With the two ribsstiffening the sheet metal and straddling the location of the hollowrivet, the hollow rivet stays in the plane of the two ribs instead ofmoving outward in response to internal fluid pressure. The formation ofthe two ribs also increases the amount of metal immediately adjacent thescore lines along the two sides of the tear strip and thus reduces theextent to which the residual webs of metal along these lines arestressed longitudinally by the internal fluid pressure. Anotheradvantage in the preferred practice of the invention is that the twoflanking ribs are dimensioned and located to serve as lip guards whenthe user drinks the beverage directly from the can.

In the preferred practice of the invention, the tab that facilitates theseverance of the tear strip functions as a secondvantage is that thehollow rivet that attaches the tab to the tear 10 Strip is spacedradially away from the region of maximum outward bowing of the can top.Another important advantage is that the offcenter location of the hollowrivet provides more room for the tab. The greater amount of availablespace for the tab makes it possible to do one of two things, either toprol vide more access room for manipulation of a tab of conventionalsize or to employ a tab that is larger than conventional size. In thepreferred practice of the invention, the tab is a ring-shaped member andthe greater space that is available makes it possible to employ arelatively large tab in which the ring opening is large enough toreceive a man's finger.

A further feature of the preferred practice of the invention relates tothe desirability of making the tear strip as easy to move as possiblewithout inviting spontaneous severance of the tear strip by fluidpressure. Spontaneous severance is a serious hazard because the abruptlyreleased tear strip is projected away from the can at high velocity.

Manual severance ofthe tear strip is facilitated by a number ofprovisions including: making the tear strip of generally triangularconfiguration, the smaller end being the inner leading end; employing arelatively small rivet to attach the tab to the leading end of the tearstrip thereby to make it possible to make the leading end of the tearstrip relatively narrow; by coining or squeezing the metal to make themetal relatively thin in the region of the leading end of the tear;scoring the metal to maximum depth around the leading end of the tearstrip to leave a residual web ofminimum thickness; scoring the metal tomoderate depth along the two longitudinal sides of the tear strip andalong the trailing edge of the tear strip.

The hazard of spontaneous severance is reduced by the two ribs inasmuchas the two ribs reduce the load in tension longitudinally of the linesof scoring along the two sides of the tear strip. The possibility ofspontaneous severance is further reduced by scoring the metal relativelylightly at the two cor- 45 preclude spontaneous severance. On the otherhand, it is easy 50 to initiate severance at the leading end of the tearstrip and the momentum of manually peeling away the tear strip readilyovercomes the relatively thick residual web at the two rearward cornersof the tear strip.

The features and advantages of the invention may be un- 55 derstood fromthe following detailed description and the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a plan view of the completed can top illustrating a 60selected practice of the invention;

FIG. 2 is a similar view of the can top prior to the final step ofmounting a tab on the tear strip;

FIG. 3 is a greatly enlarged fragment of FIG. 2 showing how the sheetmetal of the can top is squeezed in the process of 65 forming a hollowrivet, the squeezing operation resulting in an annular zone around thehollow rivet in which the outer surface of the sheet metal is depressedto a lower level with two inclined ramps providing transitions betweenthe two levels;

FIG. 4 is a transverse section along the line 4-4 of FIG. 3;

FIG. 5 is a transverse section along the irregular line 5-5 of FIG. 3;

FIG. 6 is a section along the irregular line 6-6 of FIG. 3 showing how aramp makes a transition between the two levels of the outer surfaceofthe can top;

FIG. 7 is a fragmentary plan view of a can top with a newly formedhollow rivet therein showing a pair of ribs that are formedsimultaneously with the rivet;

FIG. 8 is a greatly enlarged fragmentary cross section taken along theline 8-8 of FIG. 7 showing how a pair of dies cooperate to form therivet and the two ribs simultaneously;

FIG. 9 is a similar sectional view showing how the same two diescooperate to form other portions of the two ribs;

FIG. 10 is a view similar to FIG. 7 showing the state of the can topafter a subsequent operation in whichthe can top is scored to form atear strip and simultaneously the two previously formed ribs arereformed and enlarged;

FIG. 11 is a cross-sectional view taken along the line 11-11 of FIG. 10and showing how a pair of dies cooperate to score the can top in theregion of the previously formed hollow rivet and to reform adjacentportions of the two ribs;

FIG. 12 is an enlarged cross section along the line 12-12 of FIG. 10showing how the two dies of FIG. 11 cooperate to reform other portionsof the two ribs;

FIG. 13 is an enlarged section along the line 13-13 of FIG. 7 showingthe configuration of one of the ribs after the initial forming operationthat is carried out simultaneously with the forming ofthe rivet;

FIG. 14 is an enlarged section taken along the line 14-14 of FIG. 8 andthe line 14-14 of FIG. 10 and showing the final longitudinalconfiguration of a sheet metal rib after the rib is reformedsimultaneously with the scoring of the can top;

FIG. 15 is an enlarged portion of FIG. 2 showing how the groove or lineof scoring that forms the tear strip extends down a ramp to the lowersurface of the depressed area sur rounding the hollow rivet;

FIG. 16 is a greatly enlarged fragmentary sectional view showing how apair of dies cooperate to form the groove or score line in each of theregions where the score line extends down a ramp to the lower surfacearea, the view showing the pair of dies open in preparation for thescoring operation;

FIG. 17 is a similar view showing the two dies closed against thematerial of the can top to carry out the scoring operation;

FIG. 18 is a similar view showing the can top released by the scoringdies;

FIG. 19 is a view similar to FIG. 8 showing how a pair of dies may beemployed to form the hollow rivet and simultaneously form the two sheetmetal ribs to final configuration in one operation;

FIG. 20 is a sectional view similar to FIG. 9 showing how the same pairof dies cooperate to form other portions of the two ribs;

FIG. 21 is a cross-sectional view similar to FIG. 12 showing how a pairof dies may be employed to score the can top after the two ribs havebeen formed simultaneously with the formation of the hollow rivet;

FIG. 22 is a plan view of the tear strip on an enlarged scale; and

FIG. 23 is a sectional view of a can end with the tab removed and therivet unstaked.

In the selected embodiment of the invention shown in FIG. 1 the can top,generally designated T, has a tear strip formed by a continuous grooveor score line 32, the tear strip being of generally triangularconfiguration with straight sides. The tear strip has an innerrelatively narrow leading end that is blunt in plan configuration, theleading edge being substantially perpendicular to the longitudinal axisof the tear strip. It has been found that squaring the leading end inthis manner to form two corners and to bring the score line close to therivet at the leading end of the tear strip greatly facilitatesinitiation of severance of the tear. The tear strip has a relativelywide trailing end that is formed with rounded corners 34. A suitablesheet metal tab of a well-known type has a relatively large opening 36to receive the users finger and is connected to the tear strip 30 bymeans ofa hollow rivet 38 that is formed in the tear strip, the rivetextending through an aperture in the tab and being headed or staked intooverlapping engagement with the rim of the aperture. In the constructionshown, the tab 35 has a sheet metal tongue 40 that serves as a pliableconnection between the tab and the hollow rivet. The tab is in the formof a secondcclass lever that is of forked construction to provide twofulcrum end portions or fulcrum arms 42 which straddle the tear stripand are shaped and dimensioned to exert fulcrum force against the cantop adjacent opposite sides of the tear strip and outside of the area ofthe tear strip.

As shown in FIG. I, the can top is of a conventional crosssectionalconfiguration being formed with an outer circumferential groove 44 andan upstanding peripheral flange 45 which in the completed can isjoinedto the cylindrical body of the can to form the rim or chime ofthe can.It is to be noted in FIG. I that the tear strip is substantially shorterthan the radius of the can as measured inside the circumferential groove44 and that the inner or leading end of the tear strip is spacedsubstantially from the center of the can top, the can top center beingindicated by the numeral 46. By virtue of the tear strip 30 beingdimensioned and located in this manner, the hollow rivet 38 is spaced asubstantial distance from the center 46 of the can top.

It is apparent in FIG. 1 that the offcenter location of the hollow rivet38 also provides more room for the tab within the circumference definedby the peripheral flange 45. The inven tion takes advantage of this factby making the tab 35 larger than would otherwise be possible, the tabbeing unique in this respect in that the opening 36 is large enough toreceive a man's finger. It is also to be noted that even though the tabis relatively large, the offcenter positioning of the hollow rivet makesavailable so much space for the tab that, with the tab located on thesame diameter as the tear strip, there is a liberal clearance betweenthe handle end of the tab and the upstanding flange to provide room formanual access to the underside ofthe tab.

An important feature of the invention is the provision of a pair ofoutwardly protruding ribs 48 that are formed in the sheet metal of thecan top along opposite sides of the tear strip 30 just outside the areaof the tear strip. The two ribs 48 are in convergent positions toconform to the triangular configuration of the tear strip and extendwell beyond the leading end of the tear strip into the central area ofthe can top.

It may be seen in FIG. 1 that the two fulcrum arms 42 of the tab 35 reston the two ribs 48 respectively, the ribs strengthening the sheet metalto resist inward flexure of the can top in response to the fulcrum forceexerted by manipulation of the tab. Preferably, the inner end portionsof the two ribs 48 that lie under the fulcrum arms 42 are partiallyflattened, the flattened zones being shaded and designated by numerals50 in FIGS. 2 and 10. FIG. 14 is a longitudinal section through one ofthe ribs and shows how the flattened portion 50 of a rib 48 is lesselevation than the remainder of the rib, the partial flattening of therib reducing the extent to which the rib elevates the correspondingfulcrum arms 42 of the tab.

The hollow rivet 38 is initially formed by an operation which involvesthinning the sheet metal of the can top in an annular zone 52 around therivet, the annular zone being formed by a squeezing operation whichdisplaces the metal radially inward to form the hollow rivet. Thesqueezing opera tion also displaces the sheet metal radially outward andit is this radial outward displacement of metal that tends to increasethe extent to which the can top bulges into the head space. The initialconfiguration of the hollow rivet that results from the radially inwarddisplacement of the metal is indicated by numeral 54 in FIGS. 2-8, 10,11,15, 19 and 22.

The operation of squeezing the metal to form the hollow rivet to itsinitial configuration is preferably but not necessarily carried out insuch manner as to depress the level of the annular zone 52 relative tothe remainder of the can top. In the preferred practice of the inventionthe squeezing of the metal also forms two ramps 55 on opposite sides ofthe hollow rivet and as shown in FIG. 6, each ramp 55 provides atransition between the lower level of the depressed annular zone 52 andthe higher level of the surrounding metal ofthe can top.

It is contemplated that the hollow rivet in the can top will be formedto its initial configuration in one operation and that the can top willbe scored to form the tear strip 30 in a separate and subsequentoperation. In one practice of the invention the two ribs 48 are formedin two stages, the ribs being partially formed in one stagesimultaneously with the formation of the hollow rivet and beingcompletely formed in a second stage simultaneously with the scoringoperation. FIGS. 7 and 13 show the configuration of the ribs at the endof the first stage and FIGS 10 and 14 show the ribs at the end of thesecond stage. This particular practice of the invention will now bedescribed.

FIGS. 8 and 9 show how an upper die 56 and a lower die 58 cooperate toform the hollow rivet to its initial configuration 54 and simultaneouslypartially form the two sheet metal ribs, i.e. form the two sheet metalribs to an initial configuration. The upper die 56 has a central cavity60 which is surrounded by an annular land 62. In addition the under faceof the upper die 56 is formed with two channels 64 in which the twosheet metal ribs of the can top are to be formed to their initialconfiguration.

The lower die 58 has a shallow boss 65 that registers with the cavity 60and promotes the formation of the hollow rivet in the cavity. The lowerdie 58 is further formed with elongated projections 66 which conform tothe configuration in plan of the channels 64 of the upper die and whichoffset the sheet metal into the channels to form the partially completedribs which are designated 68 in FIGS. 7, 8, 9 and I3.

It may be noted that the elongated projections 66 of the lower die 58are somewhat angular in cross section to make the corresponding ribsangular in cross section. FIG. 8 shows how portions of the two channels64 in the upper die and corresponding portions of the elongatedprojections 66 in the lower die are relatively shallow to make the innerend portions of the two ribs 68 correspondingly shallow and FIG. 9 showshow the remainder of the channels 64 are deeper and the correspondingportions of the projections 66 are also deeper to make the outer endportions of the two ribs higher than the inner end portions.

As shown in FIG 8, the annular land 62 squeezes the sheet metal to arelatively thin thickness, the squeezing action causing the formation ofthe rivet to its initial configuration and causing the formation of thedepressed annular zone 52. The land 62 is cut away in two oppositeregions to cause the land to form the two previously mentioned ramps 55.

It may be readily appreciated that the squeezing of the metal to formthe hollow rivet also has the effect of displacing the sheet metalradially outwardly to increase the bulging of the can top. Thesimultaneous production of the partially formed ribs 68 in the can top,however, takes up metal as the metal is being spread by therivet-forming operation and does so to such extent that the formation ofthe rivet has substantially no bulging effect on the can top. Inaddition the formation of the two ribs by the first operation by virtueof drawing the sheet metal in opposite directions from the intended areaof the tear places the sheet metal in the intended area under transversetension.

The second operation in this first practice of the invention is to scorethe metal to form the tear strip 30 and to simultaneously deepen the tworibs in the can top to negate the bulging effect of the scoringoperation. This second operation is carried out by a pair of dies of thecharacter shown in FIGS. 11 and 12, the dies comprising an upper die 70and a lower die 72. The upper die 70 has a central cavity 74 dimensionedto clear the initial configuration S4 of the hollow rivet. An annularland 75 surrounds the cavity 74 to make contact with the depressedannular zone 52 of the can top and the upper die is formed with a pairof channels 76 conforming to the plan configuration of the partiallyformed ribs 68. The upper die 70 is further provided with an integralscoring element 78 with a portion of the scoring element extending alongthe annular land 75.

It is to be noted that the portions of the channels 76 of the upper die70 shown in FIG. 11 are relatively shallow to conform with thepreviously mentioned partially flattened portions 50 of the previouslydescribed completed sheet metal ribs 48. As shown in FIG. 12 theremaining portions of the two channels 76 are relatively deep to permitthe corresponding portions of the completed can top ribs 48 to berelatively high.

The lower die 72 has a pair of projections 80 to mate with the twochannels 76 of the upper die. As shown in FIG. 11 the two projections 80are relatively shallow where the completed can top ribs are to beshallow and as shown in FIG. 12, the projections are of greaterelevation where the cross sections of the completed ribs are to behigher.

It is apparent from a comparison of FIGS. 11 and 8 that the second stageof forming of the can top ribs 48 results in appreciably flattening andwidening of the inner end portions 50 of the sheet metal ribs and acomparison of FIG. 12 with FIG. 9 shows that the second stage results inboth broadening and deepening the remaining portions of the rivet. Therelieving of the transverse tension of the sheet metal and the taking upof additional sheet metal by the ribs compensate for the spreading ofthe metal in the opposite lateral directions by the scoring element 78and thus nullifies the tendency of the scoring operation to increase thebulging of the can top.

It may be seen in FIG. 11 that the scoring element 78 penetrates themetal in the depressed annular zone 52 to great depth to leave anexceedingly thin residual web but outside of the annular zone thescoring element 78 penetrates the sheet metal to lesser depth to leave athicker residual web as shown in FIG. 12. As shown in FIGS. and 15, thegroove that forms the score line extends down the two ramps 55 into thedepressed annular zone 52 to loop around the initially formed rivet 54and thereby form the leading end of the tear strip 30. The manner inwhich the upper die 70 and the lower die 72 cooperate to form thisportion of the groove 32 is indicated by FIGS. 16-18.

As best shown in FIG. 16 the lower die 72 is formed with a plateau 82that conforms with the area of the depressed annular zone 52 of the cantop and the lower die is further formed with a planar working face 84that conforms with the remainder of the can top FIG. 16 also shows howthe lower die 72 is formed with a slope 85 which is a shoulder of theplateau, there being two shoulders 85 to conform to the two ramps 55 ofthe can top. Each slope 85 forms a transition from the level of theplateau 82 to the lower level of the planar working face 84 of the diein the same manner that the corresponding ramp 55 of the can top forms atransition from the level of the metal outside the annular zone 52 tothe lower level in the annular zone. It is important to note, however,that the inclination of the slope 85 of the lower die is substantiallyless than the inclination of the corresponding ramp 55 of the can top.In this region around the leading end of the tear strip, the scoringelement 78 of the upper die 70 conforms at its lower edge with a planethat is parallel to the working surface 84 of the lower die 72.

FIG. 16 shows the upper and lower dies 70 and 72 retracted with a cantop resting on the plateau 82 of the lower die in preparation for thescoring operation. It may be noted that there is a clearance space 86between the lower surface of the can top and the lower planar surface 84of the lower die 72. When the two dies 70 and 72 are operated for ascoring operation, the two dies close towards each other with consequentflexing of the can top as shown in FIG. 17, the flexed can top followingthe inclination of the die slopes 85 and making contact with the lowerplanar working face 84 ofthe lower diev FIG. 17 shows the two dies 70and 72 at their minimum spacing in carrying out the scoring operation.It may be noted that by virtue of the inclination of each slope 85 beingless than the inclination of the corresponding ramp 55, the penetrationof the scoring element 78 into the metal progressively increases downthe length of each ramp with the result that the thickness of theresidual web 87 left by the scoring tool progressively decreases to aminimum thickness in the annular zone 52.

FIG. 18 shows how the sheet metal springs back from the planar workingface 84 of the lower die when the two dies are retracted. FIG. 18further clearly shows how the residual web 87 left by the scoring toolprogressively decreases as the residual web approaches the annular zone52.

In the preferred practice of the invention the residual web 87 formed bythe score line 32 is of minimum thickness at the leading end ofthe tearstrip and is of moderate thickness along the two longitudinal sides ofthe tear strip as well as along the trailing end of the tear strip. Ateach of the two corners 34 of the tear strip, however, the residual webis of maximum thickness. For example, referring to FIG. 22, if thenominal thickness of the sheet metal stock is 0.0145 inch the residualweb in the depressed annular zone 52 at the leading end of the tearstrip may be only 0.00350.0040 inch thick. The thickness of the residualweb increases along each of the two ramps 55 to a thickness of0.00550.0065 inch and this thickness dimension prevails along the twostraight sides of the tear strip outside of the annular zone 52. Betweenthe two lines 88 and 90 in FIG. 22 that bound each ofthe two corners 34of the tear strip, the residual web may have the relatively greatthickness of 0.009 inch to 0.012 inch and between the two lines 90 atthe trailing end of the tear strip the residual web may again be of themoderate thickness 0.00550.0065 inch.

As may be understood from the previous discussion of FIGS. 16I8, theedge of the scoring element 78 that forms the leading end of the tearstrip and the two straight sides of the tear strip is parallel to theplanar working face 84 of the lower die 72. In the region of each of thetwo corners 34, however, the striking surface of the scoring element isreduced in height to make the residual web correspondingly thick.

The fact that the residual web is exceedingly thin at the leading end ofthe tear strip greatly facilitates initiation of the severance of thetear strip by initial lifting of the ring portion of the tab 35. Oncethe leading end of the tear strip is severed the severance is continuedalong the two straight side edges of the tear strip by a single handmotion and the momentum of this hand motion easily overcomes theresistance of the thicker residual web portions at the two corners 34 ofthe tear strip. Thus the thickening of the residual web at the twocorners 34 provides the required insurance against spontaneous severanceof the tear strip by fluid pressure but at the same time does not offerundue resistance to the final manual separation of the tear strip fromthe can top.

After the hollow rivet is formed to the initial hollow configuration 54and the can top is scored with the two sheet metal ribs formed in twostages as described, the can top is ready for the final operation ofattaching the tab to the tear strip. This final operation consistsfirst, of placing the tab on the can top with the hollow rivet extendingthrough the aperture in the tongue 40 of the tab and, then, staking orspreading the outer end of the hollow rivet in a suitable manner toreform the hollow rivet into overlapping engagement with the tab.

The second practice of the invention illustrated by FIGS. 19, 20 and 21differs from the first practice in that the two can top ribs 48 aresubstantially completely formed in one operation simultaneously with theforming ofthe hollow rivet.

FIGS. 19 and 20 show an upper die 92 and a lower die 94 that cooperateto form the hollow rivet and to form the two ribs simultaneously totheir final configuration. The upper die 92 has the usual cavity 95 andthe lower die 94 has the usual shallow boss 96 to cooperate for theforming of the hollow rivet. The upper die has the usual pair ofchannels 96 and the lower die has the usual corresponding elongatedprojections 97 for cooperation with the channels to form the two can topribs 48 in one operation. As may be seen in FIG. 19, portions of thechannels 96 and of the projections 97 are relatively shallow tocooperate to form the partially flattened portions 50 of the sheet metalribs 48 and, as may be seen in FIG. 20, the remaining portions of thechannels 96 and the projections 97 are deeper to deepen the remainingportions of the sheet metal ribs. v

The second step of scoring the sheet metal to form the tear strip iscarried out by a pair of dies designated 100 and 102 in FIG. 21. The twodies 100 and 102 are similar to the previously described pair of scoringdies 70 and 72 except that the channels 103 in the upper die 100 arelarge enough to clear the previously formed sheet metal ribs 48. It isalso to be noted that the two dies confine the sheet metal with freedomfor the sheet metal to slip towards the two previously formed ribs 48.

Two factors cooperate to compensate for the spreading of the sheet metalby the scoring operation and thus prevent buckling of the can top. Onefactor is that the prior operation of forming the two ribs has drawn thesheet metal in opposite directions from the intended area of the tearstrip to leave the sheet metal across the area under a state oftransverse tension. The relief of this transverse tension by the scoringoperation partially compensates for the tendency of the scoringoperation to buckle the can top.

The second factor is that the scoring dies 100 and 102 provide ampleclearance for increase in depth of the previously formed ribs 48 and atthe same time the two dies confine the metal between the ribs to a planewith freedom for the sheet metal to shift towards the two ribs. Anytendency of the scoring operation to increase the buckle in the can topserves instead to displace metal into the two ribs.

My description in specific detail of the selected embodiments of theinvention will suggest various changes, substitu' tions and otherdepartures from my disclosure within the spirit and scope of theappended claims.

l claim: 1. A method of fabricating an easy'opening container ofthecharacter described wherein a sheet metal wall of the container isscored to form a tear strip and a tab is attached to the tear strip by ahollow rivet formed in the tear strip, said method being characterizedby the steps of:

squeezing the sheet metal wall around the base of the rivet to reducethe thickness of the wall and to form a zone in which the outer face ofthe wall is depressed to lower level and to form two ramps on theopposite sides of the zone leading to the lower level; placing one faceof the sheet metal wall against a first die having a plateau conformingto said zone and having an extensive planar working face at the base ofthe plateau, the plateau having slopes conforming to the two rampsrespectively of lesser inclination than the ramps, each slope leadingfrom the top of the plateau to said working face; positioning a seconddie against the other face of the sheet metal wall, the second diehaving a protruding scoring element to score the wall of the containerto form the tear strip, said scoring element having a leading face in aplane substantially parallel to said working face, said scoring elementbeing of a configuration to loop around the hollow rivet in the regionof said zone with two spaced portions ofthe scoring element positionedto extend from the region of the plateau across the regions of the twoslopes respectively to the region of said working face; and

causing relative movement of said dies to force said scoring elementinto the sheet metal of the can top with consequent resilient deflectionof the sheet metal against the two slopes and the working face of thefirst die to cause the scoring element to form a relatively thinresidual web in said zone with the residual web progressively increasingin thickness along the two ramps from said zone to the region outsideofthe zone.

2. A method of forming a hollow rivet in a container wall of sheetmaterial comprising:

forming a hollow rivet of sheet material integral with the containerwall;

said step of forming including squeezing a zone of sheet materialsurrounding the rivet to displace some of the material from said zoneinto the rivet and to displace other portions of the material from saidzone radially outwardly; and

offsetting the sheet material simultaneously with said step of squeezingat a region radially outwardly of said zoneto take up at least some ofsaid other portions of the material. 3. A method of forming a tearportion in a container wall of sheet material comprising:

offsetting regions of the container wall to form first and second ribsextending in the same general direction and having a section of thecontainer wall therebetween with said step of offsetting placing saidsection in tension; substantially confining the sheet material of saidsection against relative movement in a direction generally transverse tothe plane of said section and providing free space adjacent the ribs toallow enlargement of the dimensions thereof; and scoring said sectionalong a predetermined line subsequent to the offsetting operation toform the tear portion with sheet material being displaced laterally ofsaid predetermined line by the scoring operation to at least partiallyrelieve said tension in said section, the free space adjacent the ribsallowing for enlargement of the dimensions of said ribs as a result ofthe displacement of sheet material during the scoring operation. 4. Amethod of fabricating an easy-opening can top of sheet materialcomprising:

forming a hollow rivet in the sheet material by an operation thatspreads the sheet material; offsetting the sheet material simultaneouslywith the rivetforming operation to form two ribs on opposite sidesrespectively of the rivet to take up the sheet material as the sheetmaterial is displaced by the rivet-forming operation and to place asection of the sheet material between the ribs in tension; and scoringthe sheet material including said section of sheet material to form atear strip and at least partially relieve said tension in said sectionof sheet material, said step of scoring being carried out subsequent tothe rivet-forming operation, the two ribs straddling the tear strip andextending longitudinally thereof. 5. A method of fabricating aneasy-opening can top of sheet material comprising:

providing a can top of sheet material having a peripheral flangedefining a head space with the can top bowing outwardly into the headspace and with the maximum outward bowing being at the center of the cantop; forming a hollow rivet in the can top at a location spaced radiallyfrom the center of the can top to avoid locating the rivet in the regionof maximum outward bowing of the can top, said rivet being adapted toattach a second class lever to the can top; offsetting the sheetmaterial simultaneously with the rivetforming operation to form two ribson opposite sides respectively of the rivet to take up the sheetmaterial as the sheet material is displaced by the rivet-formingoperation and to place a section of the sheet material between said ribsin tension; and scoring the sheet material including said section ofsheet material to form a tear strip and at least partially relieve saidtension in said section of sheet material, said step of scoring beingcarried out subsequent to the rivet-forming operation, the two ribsstraddling the tear strip and extending longitudinally thereof, the tworibs being at locations to receive the fulcrum force of the lever and tostiffen the can top against flexure by the fulcrum force. 6. A method offorming an easyopening container wall of sheet material comprising:

forming a hollow rivet in the sheet material by an operation thatspreads some of the sheet material outwardly of the rivet; offsettingthe sheet material simultaneously with the rivetforming operation toform at least two ribs and to take up at least some of the materialdisplaced in the rivet-forming operation, the formation of said ribsplacing at least a section of the sheet material in tension;

formed simultaneously with the rivet-forming operation and an operationfor completing the forming of the two ribs being carried outsimultaneously with the scoring operation.

8. An improvement as set forth in claim 4 in which the sheet material ofthe two ribs is unrestrained during the scoring operation and metaldisplaced by the scoring operation is directed to the ribs to increasethe ribs instead of buckling the can top.

1. A method of fabricating an easy-opening container of the characterdescribed wherein a sheet metal wall of the container is scored to forma tear strip and a tab is attached to the tear strip by a hollow rivetformed in the tear strip, said method being characterized by the stepsof: squeezing the sheet metal wall around the base of the rivet toreduce the thickness of the wall and to form a zone in which the outerface of the wall is depressed to lower level and to form two ramps onthe opposite sides of the zone leading to the lower level; placing oneface of the sheet metal wall against a first die having a plateauconforming to said zone and having an extensive planar working face atthe base of the plateau, the plateau having slopes conforming to the tworamps respectively of lesser inclination than the ramps, each slopeleading from the top of the plateau to said working face; positioning asecond die against the other face of the sheet metal wall, the seconddie having a protruding scoring element to score the wall of thecontainer to form the tear strip, said scoring element having a leadingface in a plane substantially parallel to said working face, saidscoring element being of a configuration to loop around the hollow rivetin the region of said zone with two spaced portions of the scoringelement positioned to extend from the region of the plateau across theregions of the two slopes respectively to the region of said workingface; and causing relative movement of said dies to force said scoringelement into the sheet metal of the can top with consequent resilientdeflection of the sheet metal against the two slopes and the workingface of the first die to cause the scoring element to form a relativelythin residual web in said zone with the residual web progressivelyincreasing in thickness along the two ramps from said zone to the regionoutside of the zone.
 2. A method of forming a hollow rivet in acontainer wall of sheet material comprising: forming a hollow rivet ofsheet material integral with the container wall; said step of formingincluding squeezing a zone of sheet material surrounding the rivet todisplace some of the material from said zone into the rivet and todisplace other portions of the material from said zone radiallyoutwardly; and offsetting the sheet material simultaneously with saidstep of squeezing at a region radially outwardly of said zone to take upat least some of said other portions of the material.
 3. A method offorming a tear portion in a container wall of sheet material comprising:offsetting regions of the container wall to form first and second ribsextending in the same general direction and having a section of thecontainer wall therebetween with said step of offsetting placing saidsection in tension; substantially confining the sheet material of saidsection against relative movement in a direction generally transverse tothe plane of said section and providing free space adjacent the ribs toallow enlargement of the dimensions therEof; and scoring said sectionalong a predetermined line subsequent to the offsetting operation toform the tear portion with sheet material being displaced laterally ofsaid predetermined line by the scoring operation to at least partiallyrelieve said tension in said section, the free space adjacent the ribsallowing for enlargement of the dimensions of said ribs as a result ofthe displacement of sheet material during the scoring operation.
 4. Amethod of fabricating an easy-opening can top of sheet materialcomprising: forming a hollow rivet in the sheet material by an operationthat spreads the sheet material; offsetting the sheet materialsimultaneously with the rivet-forming operation to form two ribs onopposite sides respectively of the rivet to take up the sheet materialas the sheet material is displaced by the rivet-forming operation and toplace a section of the sheet material between the ribs in tension; andscoring the sheet material including said section of sheet material toform a tear strip and at least partially relieve said tension in saidsection of sheet material, said step of scoring being carried outsubsequent to the rivet-forming operation, the two ribs straddling thetear strip and extending longitudinally thereof.
 5. A method offabricating an easy-opening can top of sheet material comprising:providing a can top of sheet material having a peripheral flangedefining a head space with the can top bowing outwardly into the headspace and with the maximum outward bowing being at the center of the cantop; forming a hollow rivet in the can top at a location spaced radiallyfrom the center of the can top to avoid locating the rivet in the regionof maximum outward bowing of the can top, said rivet being adapted toattach a second class lever to the can top; offsetting the sheetmaterial simultaneously with the rivet-forming operation to form tworibs on opposite sides respectively of the rivet to take up the sheetmaterial as the sheet material is displaced by the rivet-formingoperation and to place a section of the sheet material between said ribsin tension; and scoring the sheet material including said section ofsheet material to form a tear strip and at least partially relieve saidtension in said section of sheet material, said step of scoring beingcarried out subsequent to the rivet-forming operation, the two ribsstraddling the tear strip and extending longitudinally thereof, the tworibs being at locations to receive the fulcrum force of the lever and tostiffen the can top against flexure by the fulcrum force.
 6. A method offorming an easy-opening container wall of sheet material comprising:forming a hollow rivet in the sheet material by an operation thatspreads some of the sheet material outwardly of the rivet; offsettingthe sheet material simultaneously with the rivet-forming operation toform at least two ribs and to take up at least some of the materialdisplaced in the rivet-forming operation, the formation of said ribsplacing at least a section of the sheet material in tension; scoring thesheet material along a predetermined line to form a tear portion of thedesired configuration with the scoring operation displacing materialgenerally laterally outwardly from the predetermined line to thereby atleast partially relieve said tension, at least portions of saidpredetermined line being adjacent said ribs; and further offsetting saidribs to enlarge said ribs simultaneously with the scoring operation. 7.An improvement as set forth in claim 4 in which the two ribs are formedin two stages, the two ribs being partially formed simultaneously withthe rivet-forming operation and an operation for completing the formingof the two ribs being carried out simultaneously with the scoringoperation.
 8. An improvement as set forth in claim 4 in which the sheetmaterial of the two ribs is unrestrained during the scoring operationand metal displaced by the scoRing operation is directed to the ribs toincrease the ribs instead of buckling the can top.